1. Field of the Invention
The invention relates to an apparatus and process for the vaporization of liquid precursors and deposition of a film on a suitable substrate. Particularly contemplated is an apparatus and process for the deposition of a metal-oxide film, such as a barium strontium titanate (BST) film, on a silicon wafer to make integrated circuit capacitors useful in high capacity dynamic memory modules.
2. Background of the Related Art
The increasing density of integrated circuits (ICs) is driving the need for materials with high dielectric constants to be used in electrical devices such as capacitors for forming 256 Mbit and 1 Gbit DRAMs. Capacitors containing high-dielectric-constant materials, such as organometallic compounds, usually have much larger capacitance densities than standard SiO.sub.2 --Si.sub.3 N.sub.4 --SiO.sub.2 stack capacitors making them the materials of choice in IC fabrication.
One organometallic compound of increasing interest as a material for use in ultra large scale integrated (ULSI) DRAMs is BST due to its high capacitance. Deposition techniques used in the past to deposit BST include RF magnetron sputtering, laser ablation, sol-gel processing, and chemical vapor deposition (CVD) of metal organic materials.
A liquid source BST CVD process entails atomizing a compound, vaporizing the atomized compound, depositing the vaporized compound on a heated substrate and annealing the deposited film. This process requires control over the liquid precursors and gases from introduction from an ampoule into a liquid delivery system through vaporization and ultimately to the surface of the substrate where it is deposited. The goal is to achieve a repeatable process which deposits a film of uniform thickness under the effects of a controlled temperature and pressure environment. This goal has not been satisfactorily achieved because the precursors are finicky and the deposition equipment requires a complex design.
For example, a series of problems result from the use of vaporizers. One difficulty is the lack of efficiency in vaporizing the liquid precursors. Typically, only a portion of the liquid precursors are vaporized due to low conductance in the vaporizer, thereby inhibiting deposition rates and resulting in processes which are not consistently repeatable. In addition, known vaporizers used in CVD processes incorporate narrow passages which eventually become clogged during use and are not adapted for continuous flow processes which can be stabilized. For example, U.S. Pat. No. 5,204,314 entitled, "Method for Delivering an Involatile Reagent in Vapor Form to a CVD Reactor, discloses a flash vaporizer using a matrix structure. The matrix structure generally comprises a heated screen mesh having restricted openings. After extended usage the matrix structure accumulates build up leading to a reduction in vaporization efficiency of the liquid precursors and negative effects on process repeatability and deposition rate.
Another difficulty is that BST liquid precursors have a narrow range of vaporization between decomposition at higher temperatures and condensation at lower temperatures. Known vaporizers lack temperature controlled surfaces and the ability to maintain liquid precursors at a low temperature prior to injection into the vaporizer. This results in deposition of material in the injection lines and in the vaporizer and premature condensation or unwanted decomposition of the precursors. The deposits adversely affect not only the vaporizer but also upstream components such as positive displacement pumps because the pump can rupture its pressure seals or continue to operate until the pressure relief valves on the pump are tripped. Damage to system components, of course, requires maintenance and repair and over time becomes very expensive and increases the cost of ownership of the equipment. Additionally, the deposits formed in the vaporizer may be carried downstream to corrupt other components and ultimately even be delivered to the substrate surface thereby compromising its quality. Thus, temperature controlled flow paths through the vaporizer are needed.
Still another difficulty encountered in the deposition of BST is that the deposition process is performed at elevated substrate temperatures, preferably in the range of about 400-750.degree. C. and the annealing process is performed at substrate temperatures in the range of about 550.degree.-850.degree. C. These high temperature requirements impose demands on the chambers and its other components used in the deposition process. For example, elastomeric O-rings are typically used to seal the deposition chamber and are not generally made of materials that will resist temperatures in excess of about 100.degree. C. for many fabrication cycles. Seal failure may result in loss of pressure as well as contamination of the process chemistry and the system components, thereby resulting in defective film formation on the wafer. In addition, it is necessary to prevent temperature fluctuations of vaporizer surfaces which result from thermal conduction. Loss of heat due to thermal conduction causes temperature gradients across the surface of the substrate resulting in decreased uniformity in film thickness and also increases the power demands required of the system to maintain the high temperature environment in the chamber.
There is a need, therefor, for a high conductance vaporization apparatus which can efficiently vaporize the precursors, deliver the vaporized precursors to downstream system components while maintaining elevated temperatures, preventing unwanted condensation or decomposition of precursors along the pathway and avoiding temperature gradients. It would be preferable if the system were adapted for rapid cleaning and continuous flow operation.